Npower has become the third major energy supplier to announce price hikes this winter.
The cost of electricity is set to rise 9.3% and gas 11.1% on 1 December.
SSE has also announced that it will increase prices by 8.2% from 15 November and British Gas said prices would go up by 9.2% on 23 November.
Npower blamed the rise on higher distribution and commodity costs.
New government schemes such as Eco and the Green Deal, which require energy suppliers to pay 31% more in taxes, were also a contributing factor, said Npower’s chief executive Paul Massara.
The news came as energy company Scottish Power was forced to pay £8.5m in compensation for providing misleading information to customers. Energy regulator Ofgem found that ScottishPower “did not have in place appropriate management arrangements to adequately train and monitor their doorstep and telesales agents”.
Customers who believe they may have been mis-sold to can either complete an online form or call the dedicated phone number 0845 030 3048.